A diesel particulate filter (hereinafter referred to as “DPF”), for example, is known as a filter for collecting particulate matter (hereinafter referred to as “PM”) in an exhaust gas emitted from a diesel engine. Typically, the DPF includes a number of cells that form lattice-patterned exhaust flow channels defined by partition walls of porous ceramics, and these cells are plugged alternatingly at an upstream side and a downstream side.
The DPF can only collect a limited amount of PM. Thus, upon the amount of accumulated PM reaching a predetermined amount, so-called forced regeneration for burning and removing the accumulated PM needs to be carried out. Therefore, it is desirable to precisely measure the amount of accumulated PM in order to control the forced regeneration.
For example, PATENT LITERATURE 1 discloses a PM sensor that detects an amount of accumulated PM on the basis of an electrostatic capacity (capacitance) of a capacitor formed by a pair of electrodes. The paired electrodes are disposed in a corresponding pair of cells that oppose each other with a measurement cell interposed therebetween. The measurement cell is plugged at its downstream end of the exhaust passage.